First trace around your Dew Shield to get the diameter of the mask.
Then construct 3 lines at 120 degree angles which intersect at the center
(Hint: Construct a Hexagon with compass and connect the verrtices)

Then make three triangles (size not critical)
(Hint: contsruct a line normal to the three lines, then equilateral triangles
using a compass).

They do not need to be particularly sturdy. I use file folder cardboard.

Since the openings are triangles you will get:

Far from focus - Three triangles which converge as you approach focus

At Focus six diffraction spikes that converge right at focus.

Making one of these takes about 10 minutes (after the first one)

Make one for each OTA/Dew Shield diameter
Hartman Mask